Previous studies indicated that central GABAergic mechanisms are involved in the heart rate (HR) responses at the onset of exercise. Based on previous research that showed similar increases in HR during passive and active cycling, we reasoned that the GABAergic mechanisms involved in the HR responses at the exercise onset is primarily mediated by muscle mechanoreceptors afferents. Therefore, in this study we sought to determine if central GABA mechanisms are involved in the muscle mechanoreflex-mediated HR responses at the onset of exercise in humans. Twenty eight healthy subjects (14 men) aged between 18 to 35 years randomly performed three bouts of 5-s of passive and active cycling under placebo and following oral administration of diazepam (10 mg), a benzodiazepine that produces an enhancement in GABAA activity. Beat-to-beat HR (electrocardiography) and arterial blood pressure (BP - finger photopletysmography) were continuously measured. Electromyography of vastus lateralis was obtained to confirm no electrical activity during passive trials. HR increased from rest under placebo and further increased following administration of diazepam in both passive (∆12 {plus minus} 1 beats min-1 vs. ∆17 {plus minus} 1 beats min-1, P < 0.01) and active (∆14 {plus minus} 1 beats min-1 vs. ∆18 {plus minus} 1 beats min-1, P < 0.01) cycling. Arterial BP increased from rest similarly during all conditions (P > 0.05). Importantly, no sex-related differences were found in any variables during experiments. These findings demonstrate for the first time that the GABAergic mechanisms significantly contribute to the muscle mechanoreflex-mediated HR responses at the onset of exercise in humans.